Abstract The vapour phase catalytic oxidation of 1-butene to maleic anhydride and acetic acid was investigated between 300 and 560 °C at atmospheric pressure over V 2O 5-MoO 3 catalysts. A maximum catalyst selectivity was obtained at 20 mol.% MoO 3 at 420 °C. This catalyst composition represents the optimum concentration for the evolution of reactive oxygen where higher catalyst weight reduction was detected using an electronic microbalance. X-ray analysis and IR absorption spectra showed the formation of both Mo 4V 6O 25 and Mo 6V 9O 40 phases. An oxidation mechanism is proposed for the role of both cations V 5+ and Mo 6+ in the oxidation process.